How to use builders in a loop to build array of lists of structs?

[ryansun96]

I'm trying to use a loop to build an array of lists of structs, but have been stuck with mutable borrow issues:

let mut list_builder = ListBuilder::new(StructBuilder::from_fields(FieldsOfKeyAndValue, 0));
let struct_builder = list_builder.values();

// let key_builder = struct_builder.field_builder::<StringBuilder>(0).unwrap(); //Cannot do this due to only one mutable borrow allowed at a time;
// have to use workaround below
// let value_builder = struct_builder.field_builder::<StringBuilder>(1).unwrap();
let (field_builder_key, field_builder_value) = struct_builder.field_builders_mut().split_at_mut(1);
let key_builder = field_builder_key[0].as_any_mut().downcast_mut::<StringBuilder>().unwrap();
let value_builder = field_builder_value[0].as_any_mut().downcast_mut::<StringBuilder>().unwrap();

for (k, v) in Map:
    key_builder.append_value(k);
    value_builder.append_value(v);
    struct_builder.append(true); // Compiler complains struct_builder and list_builder have more than one mutable borrows
    list_builder.append(true);
list_builder.finish()

I have seen chained calls being used elsewhere (e.g. https://github.com/apache/datafusion/blob/33b86fe02e7bbe63135995c2dbb47bf83c08143c/datafusion/core/tests/dataframe/mod.rs#L5211), however this would become very lengthy and unreadable for complex structures, like the one in https://docs.rs/arrow/latest/arrow/array/struct.StructBuilder.html . Is there a better way to avoid the multiple mut borrow issue?

for _ in 0..num_points.max(2) {
                // Add x value
                points_builder
                    .values()
                    .field_builder::<Int32Builder>(0)
                    .unwrap()
                    .append_value(rng.random_range(-10..10));
                // Add y value
                points_builder
                    .values()
                    .field_builder::<Int32Builder>(1)
                    .unwrap()
                    .append_value(rng.random_range(-10..10));
                points_builder.values().append(true);